1. Field of the Invention
The invention relates to a mercury lamp of the short arc type in which an anode and a cathode are disposed opposite one another in a silica glass arc tube which is filled with mercury and a rare gas.
2. Description of Related Art
Recently, exposure technology using a mercury lamp of the short arc type has been utilized as the light source in photolithography in the production of semiconductors and liquid crystals, and in other areas of precision machining.
A mercury lamp of the short arc type is used as a light source in this photolithography and it emits, with high efficiency, light with a wavelength of 365 nm (i-line) at which the photoresist has its sensitivity. The lamp is filled with mercury as the emission substance and with a rare gas, such as argon, krypton or the like, as the starting gas.
On the other hand, there is a mercury lamp of the short arc type in which, together with mercury, xenon gas is also added as an emission substance (for example, published Japanese Patent Application SHO 61-189636 and U.S. Pat. No. 4,732,842).
Emission from the above described mercury lamp at wavelengths around 365 nm has a broadening of a few mm. When a refraction optics system of a single type of glass is used in an exposure device, therefore, blurring of the image occurs due to chromatic aberration, causing nonuniformity of the exposure and a reduction of the image resolution. Then, the demand for the above required miniaturization can no longer be met. To correct this chromatic aberration, an achromatic lens is used in which some glass materials with different indices of refraction are combined with one another. However, since the wavelength used is short, the choice of glass materials to be used is small. Therefore, the chromatic aberration can no longer be completely eliminated.
Thus, it can be imagined that light in a narrower waveband range passes though the refraction optics system. For example, a process was proposed in which the light emitted by the lamp passes through a bandpass filter (BPF) which transmits only a certain waveband range in the vicinity of the i-line. But, even a bandpass filter has its limit of efficiency and its material is also tremendously expensive.
In order to advantageously suppress chromatic aberration using limited optical materials, it is necessary for the light to have a narrow spectral width of the i-line which is emitted by the lamp itself. Of course, the irradiance of the lamp should not be reduced. Therefore, while maintaining sufficiently high irradiance, a narrow spectral width of the i-line can be obtained.
Furthermore, a discharge lamp is known in which, not only during starting, but also during steady-state operation xenon gas is used. The mercury lamp, which is described, for example, in published Japanese Patent Application SHO 61-189636, is filled with xenon gas at pressure of up to 5 atm. In such a mercury lamp, however, besides the intense spectrum of the mercury at 365 nm, the continuous spectrum of the xenon is used. Since xenon is used, the irradiance at 365 nm (more exactly in the range of 365xc2x10.5 nm) is low. Therefore, in a mercury lamp which is filled with xenon as the rare gas, the irradiance of the i-line is inherently insufficient even if xenon is used in steady-state operation.
Therefore, a primary object of the present invention is to devise a mercury lamp of the short arc type in which, without reducing the irradiance, the spectral width of the i-line can be reduced.
Furthermore, another object of the invention is to devise an emission device for achieving the above object.
In a mercury lamp of the short arc type in which there are an anode and a cathode opposite one another in a silica glass arc tube which is filled with mercury and a rare gas, these objects are achieved in accordance with the invention in that the amount of mercury added is less than 1.0 mg/cc of the inside volume of the arc tube, and the rare gas includes at least argon (Ar) or krypton (Kr) or a mixture of these gases that is added at room temperature with a pressure of 1.0 to 8.0 atm.
By means of this arrangement, the mercury lamp of the short arc type according to the invention can emit an i-line with a narrow spectral width at a wavelength of 365 nm since the amount of mercury added is somewhat less than in a conventional mercury lamp of the short arc type. The amount of mercury in the invention is less than 1.0 mg/cc, preferably greater than or equal to 0.5 mg/cc and less than 1.0 mg/cc.
In this way, the emission of the i-line (in the range of 365xc2x10.5 nm) can be increased, and at the same time, emission can be greatly reduced in the range of 365xc2x15 nm from which the range of 365xc2x10.5 nm is extracted.
However, by reducing the amount of mercury, a new problem arises, specifically a reduction of the irradiance, if the entire lamp is considered.
In a mercury lamp of the short arc type in accordance with the invention, this defect can be eliminated by increasing the amount of argon and krypton added.
In a mercury lamp of the short arc type which is filled with xenon gas or the like as an auxiliary starting gas, the amount of xenon gas added is generally at most roughly 0.1 to 1.0 atm, i.e., it is extremely low. The reason for this is that the purpose is for it to act as a starting aid.
In view of the defect noted above relative to the known use of Xenon, not only during starting, but also during steady-state operation xenon gas, the inventors have found that even when the amount of mercury is reduced, the band region of the i-line can be made narrower and sufficient irradiance can be achieved when argon or krypton or a mixture of these gases is used instead of xenon gas as the rare gas. They have found that it is effective if specifically when adding argon or krypton alone, the pressure is 1 to 8 atm, and when adding the gas mixture, the pressure in the mixed state is 1 to 8 atm.
The above objects are achieved in accordance with one preferred aspect of the invention by argon being used as the rare gas and that the condition 0.211xe2x89xa6((Wd)xc2xd/R)xe2x89xa60.387 being met where R is half the maximum diameter of the arc tube in the radial direction in cm, d is the thickness of the arc tube in cm, and W is the input power in kW, the radial direction being defined as the direction of the cross section which is perpendicular to the axial direction which runs between the anode and the cathode.
The objects are achieved in another aspect of the invention when, using krypton as the rare gas, the condition 0.205xe2x89xa6((Wd)xc2xd/R)xe2x89xa60.418 is met.
The objects are achieved in still another aspect of the invention using a mixture of argon and krypton as the rare gas in a manner fulfilling the condition 0.209xe2x89xa6((Wd)xc2xd/ R)xe2x89xa60.387.
The invention is characterized, in one aspect, by the fact that, when argon (Ar) or krypton (Kr) at a pressure of a few atmospheres or a mixture of these gases in a given pressure range is added, given numerical values are fixed with consideration of the thickness of the arc tube (hereinafter, also called the xe2x80x9cbulbxe2x80x9d) and the like. The reason for this is that the added rare gas exerts strong influences on the thermal behavior and the arc characteristic within the arc tube when the molar ratio of the added rare gas relative to the mercury which has been added at the same time is large.
Furthermore, the objects of the invention are achieved in a UV emission device which comprises:
a mercury lamp of the short arc type in which in a silica glass arc tube there are an anode and cathode opposite one another and mercury and rare gas are added; and
a power source which supplies a given power to this mercury lamp,
by the amount of mercury added being less than 1.0 mg/cc of arc tube inside volume, by argon (Ar) being used as the rare gas with a pressure of 1.0 to 8 atm at room temperature, and by the condition 0.211xe2x89xa6((Wd)xc2xd/R)xe2x89xa60.387 being met, where R half the maximum diameter of the arc tube in the radial direction in cm, d is the thickness of the arc tube in cm, and W is the input power which is supplied by the power source to the mercury lamp in kW, the radial direction being defined as the direction of the cross section which is perpendicular to the axial direction which runs between the anode and the cathode.
The objects are achieved in another aspect of the invention in that, when using krypton as the rare gas, the condition 0.205xe2x89xa6((Wd)xc2xd/R)xe2x89xa60.418 is met.
The objects are achieved in still another aspect of the invention, when using a gas mixture of argon and krypton as the rare gas, the condition 0.209xe2x89xa6((Wd)xc2xd/R)xe2x89xa60.387 is met.